Chemically Assisted Precompression of Hydrogen Molecules in Alkaline-Earth Tetrahydrides.

Abstract

Using high-pressure diamond anvil cell experiments combined with Raman spectroscopy, X-ray diffraction, and density functional theory calculations, alkaline-earth tetrahydrides (Ca, Sr, Ba) were synthesized and characterized. These compounds contain both atomic and quasi-molecular hydrogen. The intramolecular H-H stretching frequency decreases progressively from Ca to Sr to Ba under compression, indicating that larger host cations produce longer H-H bonds. Electron localization function analysis reveals two contributing factors: charge transfer from the metal to the H2 units and steric effects imposed by the metal host lattice. The effect is most pronounced in BaH4, where at 50 GPa the H-H bond length approaches values otherwise observed in pure hydrogen only above 275 GPa, demonstrating effective chemical precompression of the molecular hydrogen units.

Mechanism

Charge transfer from the alkaline-earth metal to quasi-molecular H2 units, combined with steric effects of the host lattice, elongates the H-H bond, effectively replicating high-pressure bond lengths at substantially lower applied pressures.